In molding a special resin which lacks fluidity at a normal injection temperature, or in precision molding of products, such as lenses or optical disk substrates, which require dimensional accuracy in microns or higher, injection-compression molding is effected whereby resin injected into molds is compressed, in order to attain improvement in dimensional accuracy and uniformity in material density.
Usually, the injection-compression molding is effected by pushing out a compression pin, at a clamp mechanism section of an injection-molding machine, from a movable platen toward the molds by means of a hydraulic mechanism, thereby thrusting a pressure block toward a cavity of the movable-side mold, at a point in time corresponding to a pressure hold process after the end of the injecting operation. In injection-compression molding of this type, it is essential to control the operation timing, thrusting force, or position of the compression pin. However, in high-accuracy molding, use of a hydraulic compressor unit, as in a conventional arrangement, causes a change of the properties of the hydraulic operating fluid depending on the temperature and the like, thereby changing the follow-up performance and reproducibility of the operation of the compressor unit. Thus, it is difficult to accurately control the compression pin. Since the hydraulic compressor unit is liable to be contaminated by the hydraulic operating fluid, and is under high pressure. Moreover, it must be strictly maintained, and the unit itself is expensive.